The 802.11ax WiFi standard aims to increase data-rates by up to 1000 times. One means of doing this is with full-duplex (FD) operation, whereby a system can simultaneously transmit and receive signals on the same frequency.
Due to hardware limitations a transmitted signal will tend to leak into the receive path. At best it will degrade the signal-to-noise ratio of the receiver, and at worst the strong transmit signal will permanently damage the receiver. Spatial separation of transmit and receive antennas can isolate the paths, but only where space is available, e.g. at opposite ends of a laptop screen. For many applications where space is at a premium, like mobile phone handsets, a single antenna is more practical.
Circulators can be used to couple receive and transmit paths to a single antenna. Although circulators provide fair isolation, they are large items that do not lend themselves well to miniaturisation. At best, they only have 10% fractional bandwidth with high isolation and low loss. For example, an isolator covering 2.3-2.5 GHz can only provide 20 dB isolation over that band.
There is therefore a need for an improved means of isolating transmit and receive paths in a communication system to provide effective full-duplex operation for use in smaller systems, such as mobile phones.